1. Field of the Invention
The present invention relates to apparatus and method for sensing the speed of an electric motor and, more particularly, to an apparatus and method for indirectly sensing the speed of an universal electric motor.
2. Description of the Prior Art
The rotational speed of electric motors has typically been sensed by either direct or indirect techniques. With the direct speed sensing technique, a discrete sensor that provides an analog or digital output is operatively coupled to the motor. Discrete sensors have included tachometer-generators that provide an analog output or pick-off transducers. The pick-off transducers have included photoelectric pick-offs that cooperate with an apertured speed wheel or magnetic pick-offs that cooperates with a toothed or cogged wheel. As the apertured or toothed wheel passes through the cooperating pick-off, a digital pulse train is generated having a pulse repetition rate that is proportional to the speed of the motor. Since the direct speed sensing approach requires a discrete sensor that is operatively connected to the motor, the motor package design must oftentimes be enlarged to accommodate the sensor. Also, the requirement for a discrete sensor adds to the piecepart count and the manufacturing, assembly, and maintenance costs of the motor. The indirect speed sensing technique does not use discrete sensors and, accordingly, obviates some of the drawbacks attendant to direct speed sensing. With the indirect speed sensing approach, speed dependent current or voltage perturbations in the motor winding circuit can be measured and processed to provide an analog or digital output signal that is representative of the motor speed. In processing the current or voltage perturbations, it is oftentimes difficult to distinguish and enhance the perturbations and discriminate them from other frequency components that may be present, including the supply power frequency components and commutation and brush noise frequency components, especially during those portions of the power supply waveform near the zero axis where the magnitude of current or voltage perturbations diminishes toward zero along with the supply power waveform.
As the control requirements for electric motors become more demanding, there is an increased need for reliable and accurate motor speed sensing devices and related motor controllers that have greater capability and flexibility in their applications than prior devices so that a single indirect speed sensor can be utilized in the control of a wide class of motors.